1. Field of the Invention
This invention relates in general to casting molds and cores formed by binding foundry sand with a binder, and more particularly to the binder which can render the molds and the cores higher in strength and in sand removability after casting.
2. Description of the Prior Art
In connection with conventional production techniques for molds and cores used in casting, shell molding has been commonly used in which the molds and the cores are formed by binding foundry sand, for example, with a binder of phenolic resin regardless of the kind of alloys to be casted. Particularly, the shell molding has been frequently and effectively used for production of the cores because of superiority in productivity and dimensional accuracy.
However, in case the core produced by the shell molding is used in casting of a light alloy having a relatively low melting point such as aluminum alloy, a part of phenolic resin is subjected to thermal change under the heat of molten metal thereby to form a very rigid carbonized structure, so that the residual strength of the core after casting becomes considerably high. Accordingly, in order to facilitate disintegration of the core, the core is heated together with a resulting casting product at a high temperature such as about 500.degree. C. for a such along time as 5 to 10 hours thereby to burn out the residue of the binder which has the carbonized structure. This necessitates consumption of a large amount of energy. In this regard, it has been eagerly desired to develop binders which are easily thermally decomposable to obtain molds and cores of high disintegration characteristics. To this end, extensive development of a variety of binders offering high disintegration characteristics to molds or cores has been undertaken.
As a part of such development, investigation has been made on thermosetting resins containing no benzene ring in view of the fact that formation of the carbonized structure is due to the benzene ring of phenolic resin. However, such thermosetting resins are not sufficient in heat resistance as compared with phenolic resin and in addition are lower in hot strength. Furthermore, such thermosetting resins are too thermally decomposable, and accordingly gas defects are liable to arise when they are used for producing molds and cores, thereby lowering production yield of the molds and cores.
Moreover, from the view point of reducing the amount of energy required for heating to form the molds and cores, i.e., to solidify the binder, studies have been made to obtain mold and core forming methods in which binders can be solidified at ordinary temperature. As one of these methods, so-called cold box method has been developed in which the combination of phenolic resin composition and isocyanate compound is used as the binder for foundry sand. However, phenolic resin is used also in this method, and therefore the disintegration characteristics of molds or cores after casting is inferior.